1. Field of the Invention
The present invention relates to a magnetic recording medium, a method of manufacturing the magnetic recording medium, and a magnetic recording reproducing apparatus using the magnetic recording medium.
2. Description of the Related Art
In recent years, the range of applications of magnetic recording apparatuses such as magnetic disk drives, flexible disk drives and magnetic tape apparatuses has markedly increased, and the importance of such apparatuses has increased. Also, the recording density of magnetic recording media which are used in such apparatuses is now increasing greatly. In particular, a steeper increase in surface recording density followed the introduction of a magneto-resistive (MR) head and a partial response maximum likelihood (PRML) technique. Also in recent years, the introduction of a giant magneto-resistive (GMR) head and a tunneling magneto-resistive (TuMR) head has led to significant increases in the surface recording density of magnetic recording media.
In addition, there is a demand for achieving a high coercive force, a high signal-to-noise ratio (SNR) and a high resolution of a magnetic recording layer in order to achieve a further increase in the recording density of magnetic recording media in the future.
In a widely used longitudinal magnetic recording system, self-demagnetization, that is, recording magnetic domains adjacent to each other in a transition region of magnetization weakening each other's magnetization, becomes dominant as linear recording density is increased. Accordingly, in the longitudinal magnetic recording system, it is necessary to make a magnetic recording layer thin in order to avoid self-demagnetization and increase shape magnetic anisotropy. However, when the film thickness of a magnetic recording layer is reduced, the magnitude of an energy barrier for maintaining magnetic domains and the magnitude of thermal energy become so close in level to each other that a phenomenon (thermal fluctuation phenomenon) in which the recorded amount of magnetization is relaxed due to the influence of temperature cannot be ignored. This is regarded as a determinant of the linear recording density.
In such circumstances, an anti-ferromagnetic coupling (AFC) medium has recently been proposed as a technique to meet the demand for improving the linear recording density in the longitudinal magnetic recording system, and efforts are being made to avoid the thermal magnetic relaxation problem with longitudinal magnetic recording.
A perpendicular magnetic recording system is attracting attention as a promising technique for achieving a further increase in surface recording density. Whilst a known medium is magnetized in an in-plane direction in the conventional longitudinal magnetic recording system, the perpendicular magnetic recording system is characterized by magnetization in a direction perpendicular to a medium surface. Accordingly, the perpendicular magnetic recording system is thought to be more suitable for high-density recording because it is possible to avoid the influence of self-demagnetization, which is a hindrance to achievement of high linear recording density in the longitudinal magnetic recording system. In addition, the perpendicular magnetic recording system is thought to be comparatively unsusceptible to thermal magnetic relaxation, which is a problem with longitudinal magnetic recording, because it is possible to maintain a certain magnetic layer thickness.
In ordinary cases, when a perpendicular magnetic recording medium which uses a perpendicular magnetic recording system is manufactured, an orientation control layer, a magnetic recording layer, and a protective layer are formed in this order on a non-magnetic substrate. In some cases, a lubricating layer is applied on a surface of the protective layer. Moreover, in general, a soft magnetic film called an underlayer is formed under the orientation control layer. In the orientation control layer, a seed layer and an intermediate layer are laminated in order from the substrate side. The intermediate layer is formed for the purpose of improving the characteristics of the magnetic recording layer. The seed layer is regarded as having the functions of aligning crystals in the intermediate layer and the magnetic recording layer and of controlling the shape of the magnetic crystals.
The crystal structure of the magnetic recording layer is important in order to manufacture a perpendicular magnetic recording medium having excellent characteristics. That is, in many cases, a hexagonal close-packed (hcp) structure is employed as the crystal structure of a magnetic recording layer in a perpendicular magnetic recording medium. However, it is important that a hexagonal close-packed (hcp) (002) crystal plane be parallel to a substrate surface, in other words, that crystal c-axis [002] axes be aligned in the perpendicular direction with as little disturbance as possible.
However, whilst a perpendicular magnetic recording medium has an advantage in that a comparatively thick magnetic recording layer can be used, the total film thickness of all the laminated thin films in the medium tends to increase in comparison with the current longitudinal magnetic recording medium, so there is a disadvantage in that the medium layer laminating process includes a factor responsible for disturbance in the crystal structure.
Conventionally, in order to align crystals in a magnetic recording layer with as little disturbance as possible, Ru, which employs an hcp structure, has been used as an intermediate layer in perpendicular magnetic recording media, in the same way as in a magnetic recording layer. Since crystals in the magnetic recording layer are epitaxially grown on a Ru (002) crystal plane, a magnetic recording medium having excellent crystal orientation is obtained (see, for example, JP-A 2001-6158).
In addition, a seed layer positioned under an intermediate layer is required to improve the crystal orientation of the intermediate layer. For this, a seed layer employing a face-centered cubic (fcc) structure has been used (see, for example, JP-A-2002-109720). In this case, a (002) crystal plane which has an hcp structure in an intermediate layer is preferentially oriented on the orientation of a (111) crystal plane which has an fcc structure. In this manner, the total film thickness for obtaining the same orientation can be reduced more than in the case in which Ru, as an intermediate layer, is directly formed on an underlayer.
JP-A-2005-190517 describes that Ti, Au-50Cu having an hcp structure is used for a first intermediate layer, Ag-40Cu having an fcc structure is used for a second intermediate layer, and Ru is used for a third intermediate layer on an underlayer.
JP-A-2004-46990 describes that a material of a composition employing a C11b structure is used as an orientation control film. As the orientation control film employing the C11b structure, a material including at least one or two or more kinds of Al, Ag, Au, Cu, Ge, Hf, Ni, Si, Ti, Zn and Zr are described.
However, the conventional seed layer is insufficient to obtain a perpendicular magnetic recording medium having excellent recording and reproducing characteristics. A perpendicular magnetic recording medium is desired which is capable of maintaining writing ability at the time of recording by reducing the film thickness between a soft magnetic underlayer and a magnetic recording layer; achieving an improvement in the crystal orientation of the magnetic recording layer and the uniformity and reduction in the crystal grain diameter; and being easily manufactured.
For example, as in JP-A-2002-109720, it is necessary to thickly set the film thickness of a seed layer to a certain thickness (10 nm or greater) in order to improve the crystal orientation of a magnetic recording layer when a material having an hcp structure is used for the seed layer. However, when the seed layer is made of a non-magnetic material and the film thickness of the seed layer is increased, the distance between the magnetic recording layer and a soft magnetic underlayer is increased, thus weakening the attraction of magnetic flux from a recording head at the time of recording and reducing writing ability.
On the other hand, when a material having an fcc structure is used as the seed layer, the crystal orientation of the magnetic recording layer can be improved to a certain level even if the film thickness is about 5 nm. However, when the material having an fcc structure is used as the seed layer, a problem occurs in that the crystal grain diameter of the seed layer is increased along with the improvement in the crystal orientation of the magnetic recording layer. In general, one crystal in an intermediate layer and one crystal in a magnetic recording layer are sequentially grown on one crystal of a seed layer, and thus an increase in the crystal grain diameter of the seed layer means an increase in the crystal grain diameter of the magnetic recording layer. When a crystal grain diameter of the magnetic recording layer is increased, a recording bit transition border is not formed in a smooth straight line and this causes problems when improving recording density. In this manner, when a seed layer having an fcc crystal structure is used, it is difficult to achieve both an improvement in the crystal orientation of a magnetic recording layer and uniformity and reduction in the crystal grain diameter of the magnetic recording layer.
However, in order to further improve surface recording density, it is essential to achieve high crystal c-axis orientation in the magnetic recording layer and uniformity and reduction in the crystal grain diameter of the magnetic recording layer.